Function acting as both decorator and context manager in Python?

Question

This might be pushing things a little too far, but mostly out of curiosity..

Would it be possible to have a callable object (function/class) that acts as both a Context Manager and a decorator at the same time:

def xxx(*args, **kw):
    # or as a class

@xxx(foo, bar)
def im_decorated(a, b):
    print('do the stuff')

with xxx(foo, bar):
    print('do the stuff')

Asked By: Jacob Oscarson

||

Source

Answer 1

Starting in Python 3.2, support for this is even included in the standard library. Deriving from the class contextlib.ContextDecorator makes it easy to write classes that can be used as both, a decorator or a context manager. This functionality could be easily backported to Python 2.x — here is a basic implementation:

class ContextDecorator(object):
    def __call__(self, f):
        @functools.wraps(f)
        def decorated(*args, **kwds):
            with self:
                return f(*args, **kwds)
        return decorated

Derive your context manager from this class and define the __enter__() and __exit__() methods as usual.

Answered By: Sven Marnach

Answer 2

Here’s an example:

class ContextDecorator(object):
    def __init__(self, foo, bar):
        self.foo = foo
        self.bar = bar
        print("init", foo, bar)

    def __call__(self, f):
        print("call")
        def wrapped_f():
            print("about to call")
            f()
            print("done calling")
        return wrapped_f

    def __enter__(self):
        print("enter")

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("exit")

with ContextDecorator(1, 2):
    print("with")

@ContextDecorator(3, 4)
def sample():
    print("sample")

sample()

This prints:

init 1 2
enter
with
exit
init 3 4
call
about to call
sample
done calling

Answered By: jterrace

Answer 3

class Decontext(object):
    """
    makes a context manager also act as decorator
    """
    def __init__(self, context_manager):
        self._cm = context_manager
    def __enter__(self):
        return self._cm.__enter__()
    def __exit__(self, *args, **kwds):
        return self._cm.__exit__(*args, **kwds)
    def __call__(self, func):
        def wrapper(*args, **kwds):
            with self:
                return func(*args, **kwds)
        return wrapper

now you can do:

mydeco = Decontext(some_context_manager)

and that allows both

@mydeco
def foo(...):
    do_bar()

foo(...)

and

with mydeco:
    do_bar()

Answered By: nosklo

Answer 4

In Python 3.2+, you can define a context manager that is also a decorator using @contextlib.contextmanager.

From the docs:

contextmanager() uses ContextDecorator so the context managers it creates can be used as decorators as well as in with statements

Example usage:

>>> from contextlib import contextmanager
>>> @contextmanager
... def example_manager(message):
...     print('Starting', message)
...     try:
...         yield
...     finally:
...         print('Done', message)
... 
>>> with example_manager('printing Hello World'):
...     print('Hello, World!')
... 
Starting printing Hello World
Hello, World!
Done printing Hello World
>>> 
>>> @example_manager('running my function')
... def some_function():
...     print('Inside my function')
... 
>>> some_function()
Starting running my function
Inside my function
Done running my function

Answered By: Mark Amery

Answer 5

Although I agree (and upvoted) @jterrace here, I'm adding a very slight variation that returns the decorated function, and includes arguments for both the decorator and the decorated function.

class Decon:
    def __init__(self, a=None, b=None, c=True):
        self.a = a
        self.b = b
        self.c = c

    def __enter__(self):
        # only need to return self 
        # if you want access to it
        # inside the context
        return self 

    def __exit__(self, exit_type, exit_value, exit_traceback):
        # clean up anything you need to
        # otherwise, nothing much more here
        pass

    def __call__(self, func):
        def decorator(*args, **kwargs):
            with self:
                return func(*args, **kwargs)
        return decorator

Answered By: openwonk

Function acting as both decorator and context manager in Python?

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